The invention relates to a process for purification of trimellitic acid anhydride. From U.S. Pat. No. 2,888,465 it is known that trimellitic acid and acetic acid anhydride can be reacted in the presence of normally liquid alkylbenzenes, such as, toluene or xylene, at a temperature between 80.degree. and 135.degree.C, preferably 95.degree. to 150.degree.C, until a homogeneous liquid phase is formed. From this liquid phase the trimellitic acid anhydride can be obtained. However, it appears desirable to improve the yield of this reaction and the quality of the acid anhydride which is obtained in several crystal fractions after cooling.
The patent also mentions the use of benzene as solvent provided the reaction is carried out at an elevated temperature and elevated pressure. However, in this case, the formation of a homogeneous liquid phase is likewise necessary. Therefore and in view of the very low solubility of trimellitic acid anhydride in benzene even at elevated temperature large amounts of benzene and a temperature above 135.degree.C are necessary. Both of these circumstances are undesirable since they result in an impairment of the quality of the product and of the yield.